1. Field of the Invention
The present invention relates to a method of producing a polymer capacitor and a polymer capacitor produced thereby. More particularly, the present invention pertains to a method of producing a polymer capacitor, in which micropores containing an electrolytic solution are formed in an ion exchange membrane, and a polymer capacitor having improved electrical properties produced thereby.
2. Description of the Prior Art
With the spread of IT products, such as mobile phones, notebook computers, and PDAs, which are handy to carry, a Ni-MH (nickel-metal hydride) secondary battery conventionally used has been replaced with a lithium ion secondary battery which has a single cell voltage three times as higher and a capacitance rather larger than in the conventional secondary battery. Due to voltage properties, depending on the type of device, as well as lower price and more stable quality than the lithium ion battery, the use of Ni—MH has continued. However, a demand for high capacitance in the same size is growing and development of the lithium ion secondary battery is accelerating because of diversified functions of the devices and a boom in development of portable devices.
With respect to the secondary battery, many studies are being conducted towards improvement of capacitance of the lithium ion battery, slimness of the lithium ion battery, improvement of performance of a lithium ion battery, which can be manufactured in a variety of different forms and development of a novel polymer battery as a next generation battery.
Meanwhile, as an example of the next generation battery, an electric double layer capacitor having electrical properties that are equal to the conventional lithium ion battery has been developed.
Additionally, another type of electric double layer capacitor has been developed, in which surface areas of electrodes are enlarged to increase an energy density. This capacitor has the energy density of 50-75 Wh/kg that is five to ten times as high as the conventional electric double layer capacitor.
A traditional secondary battery usually discharges in a level of 60-80% of the total capacitance to prolong a charge/discharge cycle life thereof. For example, a practical mass energy density of a lithium ion secondary battery having a mass energy density of 100 Wh/kg is 70 Wh/kg.
On the other hand, since the electric double layer capacitor can discharge almost completely, it has an energy density that is substantially equal to the lithium ion secondary battery. Unlike the electric double layer capacitor which employs a surface area of carbon for improvement, effort has been made to develop a polymer capacitor having a novel structure which includes metal electrodes and solid electrolytes. This is characterized in that it includes ion exchange resins and metal electrodes having a very large surface area, and has a high capacitance.
Since the metal electrodes have no active functional groups existing on a surface of carbon, an internal voltage may be increased to 2.5-6 V. Furthermore, pores may be formed in an ion exchange membrane to increase the energy density to twice as high or more as the lithium ion battery, and to assure a charging time of 1 min and a semi-permanent life. Accordingly, it is possible to use a capacitor as an energy source surpassing the performances of batteries. This exceeds the notion of the traditional capacitor.
A capacitance, namely, a performance of the polymer capacitor, depends on how many lithium ions can be contained in an electrolytic membrane. However, since there is a limit in a saturated concentration of ions capable of being contained in an electrolytic solution, a pool for collecting the electrolytic solution and then storing it, that is, pores may be formed in the electrolytic membrane so as to enable the electrolytic membrane to assure a very high capacitance in a great quantity. However, in such a case, stable electrical properties such as energy density and capacitance can be gained only when sizes, shapes and distribution of the pores are optimized.
Conventionally, a process has been adopted to form pores in an electrolytic membrane of a polymer capacitor, in which a metal chelate is adsorbed onto an ion exchange resin and reduced to deposit a metal on a surface of the resin to form: metal electrodes, and voltage is then applied to the metal electrodes while the resulting structure is dipped in an electrolytic solution.
In the above process, the pores are formed due to electrolysis of water contained in the resin. At this time, it is known that sizes or the number of the pores are adjusted by controlling the applied voltage and pressure. Another process of forming pores by rapidly evaporating water from a surface of a membrane is also known. However, it is believed that it is difficult to control the number or sizes of the pores through this process.